Their case for this massive invasion of privacy? To keep taxes low, of course!
Which is very obviously the usual trap of "we must do something! This is something! So we must do this!" That level of thinking is obviously why Treasury pays external consultants the big bucks.

But it gets worse. Because Statistics New Zealand's "big brother" Integrated Data Infrastructure isn't enough for them. There are gaps from people who haven't been forced to interact with the government yet, and the data isn't integrated enough. So they're pushing for a national identity register and national citizen numbers to "capture the entire population" and ensure they can keep track of us all properly. At this stage, there is the only mention of privacy in the entire report, where (after highlighting that New Zealanders value privacy and are likely to react to their idea with "disdain and distrust") they ominously note that "a culture shift is required" away from valuing personal privacy.

So there you have it: the privacy of the poor must be sacrificed to ensure freedom from taxation for the rich. And that's why StatsNZ can tell whether you've had an abortion and the current balance of your student loan.